1. Field of the Invention
This invention relates to a transmitter, a receiver, a communication method and a radio communication system which are suitably applied to, for example, a portable telephone system in which Identification (ID) information of mobile stations is transmitted and received.
2. Description of the Related Art
Conventionally, in this type of radio communication systems, an area to which a communication system is provided is divided into cells of a desired size, and a base station is installed in each of the cells as a stationary radio station such that a portable telephone as a mobile station radio-communicates with a base station in a cell where the portable telephone exists. This radio communication system comprises, for example, portable telephones and base stations in a portable telephone system, wherein each of the portable telephones and the base stations is equipped with a transmitter and a receiver which are used for communications from a portable telephone to a base station as well as communications from a base station to a portable telephone.
In this event, when a radio communication between a portable telephone and a base station is made in accordance with a predetermined communication scheme, a transmitter side encodes an information bit sequence to be transmitted, modulates it in accordance with a predetermined scheme to generate an encoded symbol sequence, and transmits the encoded bit sequence through a predetermined transmission line as a transmission symbol sequence. In case that any symbol errors occur on the transmission line by any reason, a receiver side detects such errors in a received reception symbol sequence and then corrects the errors in accordance with a predetermined scheme to restore a correct information bit sequence.
For performing error detection and correction, for example, in a portable telephone system, error correcting redundancy bits, such as Cyclic Redundancy Check (CRC), Bose Chaudhuri Hocquenghem code (BCH), Reed Solomon code (RS-Code), or the like are added for the transmission of relatively short information bit sequences of several bits. Specifically, several bits of redundancy bits are added to a short information bit sequence of several bits in the transmitter to increase the distance between bit information (bit sequences). In this way, even if symbol errors occur in the process of transmission through a transmission line, since the distance between the bit sequences is extended, a receiver can correct errors in received reception symbol sequences up to several bits and restore the information bit sequence. Hence, an error ratio is reduced.
In this connection, the distance between bit sequences is defined by the number of bits having different values between a bit sequence W.sub.0 and a bit sequence W.sub.1, where the bit sequence W.sub.0 is derived by adding several bits of redundancy bits to a relatively short information bit sequence of several bits, and the bit sequence W.sub.1 is corrupted from the bit sequence W.sub.0 due to errors which may occur in the transmission through a transmission line.
In this event, the receiver is configured to decode information bit sequences by treating reception symbol sequences as a group of binary signals having a value "0" or a value "1." Such an approach is generally called "hard determination decoding." However, since an actual receiver does not receive symbol sequences as binary signals each having "0" or "1." the receiver sets a certain value as a threshold such that the receiver determines whether each value of symbols in the reception symbol sequences is "0" or "1" after comparing each value with the threshold.
In such a processing step, if the value of each symbol in a reception symbol sequence is close to the threshold, a very subtle determination should be made as to whether the value of each symbol is regarded as "0" or "1." On the other hand, a reception symbol sequence also includes symbols having values which can be apparently determined to be "0" (or "1"). However, in the reception side, since the decoding is performed on reception symbol sequences after the determination of each symbol value being "0" or "1," symbols requiring subtle determination and apparently determinable symbols are uniformly treated. It is therefore hard to say that a highly accurate error correction is implemented.
On the contrary, another approach called "soft determination decoding" has been proposed. This approach employs convolutional encoding and Viterbi decoding to determine reception symbol sequences as multi-value signals for identification and restore information bit sequences. For example, a transmission symbol sequence generated by convolutional encoding an information bit sequence to be transmitted by an encoder in the transmission side is received and decoded as a reception symbol sequence by a Viterbi decoder in the reception side. In this event, since the Viterbi decoder receives, as an input reception symbol sequence, the values of the reception symbol sequence as they are, symbols unclearly determined to be "0" or "1" are distinguished from symbols clearly determined to be "0" or "1." Therefore, in all likelihood sequence estimation can be performed with high precision in general.
The convolutional encoding and the Viterbi decoding are generally used for transmission and reception of relatively long information bit sequences typically ranging from approximately 50 to 100 bits, and are not often used for transmission and reception of short information bit sequences. However, even when the convolutional encoding and Viterbi decoding are used for transmission and reception of short information bit sequences, in all likelihood sequence estimation can be performed with high precision in the reception side by adding tail bits to an information bit sequence to be transmitted in the transmitter side to increase the distance between bit sequences.
Actually, when a long information bit sequence is convolutional encoded for transmission in the transmission side, and Viterbi decoded in the reception side, the proportion of added tail bits is smaller as the information bit sequence is longer so that adverse effects due to the addition of tail bits are not necessary to consider seriously. However, in case that a short information bit sequence such as ID information of a portable telephone is convolutional encoded for transmission and Viterbi decoded in the reception side, that is, the information bit sequence to be transmitted includes merely several bits, since the information bit sequence is convolutional encoded together with the tail bits added thereto, the number of symbols of the symbol sequence is further increased. As a result, a problem arises in that a longer time is required for decoding the symbol sequence in the reception side.